1. Key Laboratory of High-speed Railway Engineering of the Ministry of Education, Southwest Jiaotong University, Chengdu, P. R. China
2. Department of Forest Engineering, Resources and Management, Oregon State University, Corvallis, OR, USA
3. Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing, P. R. China; also Jiangsu Province's Geotechnical Research Center, Nanjing, P. R. China
4. Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing, P. R. China; also Jiangsu Province's Geotechnical Research Center, Nanjing, P. R. China.